Relation pm#author (description,causal_entity)
  subtype:  main_author  co-author
  equal:  has_author (pm)
  supertype:  relation_from_description_content/medium/container (description_content/medium/container,*)
     supertype:  relation_from/to_thing_of_common_kind (*)  this type permits to categorize relations according to their signatures and hence offers (i) a concise way to set essential exclusion relations, and (ii) a systematic and easy-to-follow categorization
        >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
  supertype:  Creator (entity,entity)  to specify an entity primarily responsible for making the content of a resource
     supertype:  creator (entity,entity)
        supertype:  attributive_relation__attributiverelation (*)  like pm#binary_relation, this type mostly exists to categorize what cannot be categorized elsewhere
           supertype:  relation_playing_a_special_role (*)  this type permits to categorize relations according to their roles ; this is a traditional but quite subjective way of categorizing relations
              >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
        supertype:  who_relation__whorelation (*)
           supertype:  wh-/how_relation (*)  this type permits to categorize relations according to the usual who/what/why/where/when/how questions ; this is a traditional but very subjective and ineffective way of categorizing relations 
              >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type